Two-dimensional (linear) quadrupole ion traps are devices in which ions are introduced into or formed and contained within a trapping volume formed by a plurality of electrodes or rod structures by means of substantially quadrupolar electrostatic potentials generated by applying RF voltages, DC voltages or a combination thereof to the electrodes.
It is a constant challenge in manufacturing to maintain high yield rates for two-dimensional ion traps without compromising ion trap performance. The performance of an ion trap depends upon many things, including the structure of the ion trap itself, and its mode of operation, for example.
When using a mass selective instability scan in a two-dimensional ion trap, the ions are most efficiently ejected from the trap in a radial direction through an aperture in one or more of the electrodes (although some researchers have ejected ions between two of the quadrupole electrodes). When an aperture (or apertures) is cut into one or more of the two-dimensional ion trap electrodes to allow ions to be ejected from the device, the electric potentials are degraded from the theoretical quadrupole potential and therefore the presence of this aperture can impact several important performance factors.
The introduction of an aperture into a two-dimensional ion trap not only may degrade the theoretical quadrupole potential, but may also contribute to the degradation of the structural integrity of the rods themselves, thus leading to mechanical deviations in the axial direction (the direction substantially parallel to the length of the electrodes) and ultimately affecting the performance characteristics such as the resolution attainable by such an ion trap when used as a mass spectrometer.
The performance of a two-dimensional ion trap is more susceptible to mechanical errors than a three-dimensional ion trap. In a three-dimensional ion trap, all of the ions occupy a spherical or ellipsoidal space at the center of the ion trap, typically an ion cloud of approximately 1 mm in diameter. The ions in a two-dimensional ion trap, however, are spread out along a substantial fraction of the entire length of the ion trap in the axial direction which can be several centimeters or more. Therefore, geometric imperfections, misalignment of the rods, or the mis-shaping of the electrodes can contribute substantially to the performance of the two-dimensional ion trap. For example, if the quadrupole electrodes are not parallel along the substantial length of the electrodes, then ions at different axial positions within the ion trap experience slightly different field strength and therefore have slightly different q values. This variation in q value will in turn cause ejection times during mass analysis which are dependent on the ion respective axial position. The result is increased overall peak widths and degraded resolution.
As indicated above, one reason for the rejection of two-dimensional ion traps after it has been manufactured is its poor resolution during operation. Resolution for a two-dimensional ion trap is typically specified in terms of peak width (resolution=mass/peak width).
In addition to mechanical errors causing axial field inhomogeneity, the fringe fields caused by the end of the electrodes as well as the ends of any slots cut into the electrodes can also cause significant deviation in the strength of the radial quadrupole field along the length of the device. Ideally to keep the electric fields uniform, the ejection aperture would extend along the entire length of the electrode, but this presents numerous construction challenges. To avoid these, ejection slots are typically located only along some fraction of the central region (for example 60%) of the total ion trap length. This however leads to a variation in the radial quadrupolar potential near the ends of the slots in addition to the effects at the ends of the rods. Ions which reside in these areas are therefore ejected at different times than ions residing more in the center of the device and this again can result in a reduction in mass resolution.
It is known that the resolution for such devices can be improved by utilizing a large axial trapping field. This can be seen in FIG. 1, trace 105, which shows the axial potential as a function of axial position (the position of the ion cloud along the axial direction of the ion trap). A large axial trapping field reduces the axial spread of the ion cloud, compressing the cloud so that it experiences fewer field inhomogeneities. This enables a smaller variation in q values to be obtained and results in better resolution. Unfortunately, compression of the ion cloud simultaneously increases space charge induced mass shifts. This also compromises ion storage volume or space charge capacity for this device. Ultimately, altering the axial potential in this manner compromises between resolution and space charge capacity.
There is a need for an improved two-dimensional ion trap and a method of operating such a two-dimensional ion trap which enhances the resolution whilst producing a minimal impact on space charge capacity.